Repeat After Me

When zapped twice with laser beams in just the right way, atoms can “remember” precisely what they've done before, physicists report. That ability could be key for sending uncrackable coded messages over long distances.

To develop ultrasecure lines of communication, physicists hope to use the quantum state of photons to convey information that can only be interpreted by the intended receiver. But photons will travel only so far before they are absorbed by the air or the fiber-optic cable through which they're traveling. To send quantum messages great distances, researchers need relay stations known as quantum repeaters, which convey the quantum state of photons from station to station. Such repeaters must be able to remember dollops of quantum information.

A simple gas of rubidium atoms can provide that memory, report Mikhail Lukin and colleagues at Harvard University. The researchers zapped the gas with two lasers. The first pulse caused electrons in some of the atoms to flip the orientation of their magnetic poles. While flipping, the electrons gave out a flash of photons. The second pulse boosted the flipped electrons into a higher-energy "excited" state. From there they quickly dropped back into their original, unflipped "ground" state. As they fell, the electrons emitted a second flash of photons. Most importantly, the number of photons in the two pulses is strongly correlated, they report online today in Science. When the second laser pulse arrived, the individual rubidium atoms remembered how they responded to the first laser pulse.

The experiment demonstrates the memory necessary for a quantum repeater, says Ignacio Cirac of the Max Planck Institute for Quantum Optics in Garching, Germany, who notes that similar results have just been obtained by Jeff Kimble and colleagues at the California Institute of Technology in Pasadena. However, a practical repeater is still a ways off, Cirac says: "This is a fundamental step, but there are a lot of technological steps remaining."